将类列表添加到字典C＃中

Question

I have a list of my class

List<Example> exampleList

Which already has all the data inside of it. I need to create a dictionary

Dictionary<string, List<Example>> exampleDictionary

The Key needs to be Example.Name and the value needs to Example

Here is my code below. The problem is Example.Name can be the same. I need to group by Name. I need to loop through my list and if the Name does not exist add new Key and Value otherwise add the Value to the Key. I know I am setting this up wrong but I can't seem to figure out the correct way of doing this.

foreach(var x in exampleList)
{
   if(!exampleDictionary.ContainsKey(x.Name)
      exampleDictionary.Add(x.Name, x)
   else
      exampleDictionary[x.Name] = x;       
}

I know this code wouldn't build. I am not sure how to set this up.

Answer 1

You can use LookUp() extension method:

var lookup = exampleList.ToLookUp(e => e.Name);

This method returns a Lookup<string, Example> , a one-to-many dictionary that maps keys to collections of values.

But your code can be fixed grouping by Name and adding each group to exampleDictionary :

foreach (var g in exampleList.GroupBy(e => e.Name))
    exampleDictionary.Add(g.Key, g.ToList());

Or

var exampleDictionary = exampleList.GroupBy(e => e.Name).ToDictionary(g => g.Key, g => g.ToList());

Answer 2

This should work

Dictionary<string, List<Example>> exampleDictionary = new Dictionary<string, List<Example>>();

foreach(var x in exampleList)
{
   if(!exampleDictionary.ContainsKey(x.Name)) {
      exampleDictionary[x.Name] = new List<Example>();
   } 
   exampleDictionary[x.Name].Add(x);       
}

Answer 3

You can also use ToDictionary extension method to achieve what you want:

Dictionary<string, List<Example>> exampleDictionary=exampleList.GroupBy(e => e.Name)
                                                               .ToDictionary(g => g.Key,g.ToList());

Answer 4

Basically the same as user469104 (+1)

List<Example> le = new List<Example>() { new Example("one"), new Example("one"), new Example("two") };
Dictionary<string, List<Example>> de = new Dictionary<string,List<Example>>();
foreach (Example e in le)
{
    if (de.ContainsKey(e.Name))
        de[e.Name].Add(e);
    else
        de.Add(e.Name, new List<Example>() { e });
}